Adhesive for optical disk and optical disk

ABSTRACT

The present invention provides an adhesive that can provide an excellent durability (reliability) to a bonded optical disk having a translucent reflection film comprising silicone, silver or a silver alloy. The adhesive of the present invention contains a (meth)acrylate monomer (A) having a solution resistance of 1.0×10 10  Ω·cm or more.

TECHNICAL FIELD

The present invention relates to an adhesive for an optical disk, inparticular, an adhesive for an optical disk capable of being cured byirradiation of an ultraviolet ray, and an optical disk bonded with theadhesive, in particular, an optical disk formed by bonding two disksubstrates, represented by DVD.

BACKGROUND ART

Under present circumstances, a DVD bonded type optical disk includes, interms of the constitution of the recording layer, DVD-5 having arecording capacity of about 5 GB with a single recording layer and DVD-9having a recording capacity of about 9 GB with two recording layers, andDVD-9 having a larger recording capacity is currently being mainstream.The substrate of DVD-9, which is the current mainstream, uses analuminum compound as a total reflection film, and gold as a translucentreflection film. Since the translucent reflection film must transmitlaser, it must be a thin film comparing to the total reflection film,and thus gold, which is a representative example of stable compounds,has been used.

However, since gold is an expensive material, it is being consideredthat the material of the translucent reflection film is replaced with asilicone compound and a silver compound, which are inexpensive. In orderto further increase the recording capacity, investigations for bluelaser are being currently advanced. In the case of red laser, there isno problem in transmittance of red laser when the translucent reflectionfilm is gold, silicone, silver or a silver alloy. In the case of bluelaser, however, it has a wavelength around 400 nm, and silver exhibitsexcellent performance in transmittance at that wavelength comparing tothe other materials. Therefore, a translucent reflection film usingsilver is being developed. However, silver is liable to be oxidizedcomparing to gold, and is chemically unstable. Therefore, a bonded diskusing a silver translucent reflection film cannot provide suchdurability (reliability) that is equivalent to the conventional bondedoptical disk using gold as a translucent reflection film. Theimprovement in durability (reliability) is a significant problem forattaining full-fledged practical application of the silver translucentreflection film.

The invention is to solve the problem with improvement of an adhesive,and an object thereof is to provide an excellent adhesive that canprovide such an optical disk that has high durability (reliability)equivalent to the conventional optical disk using a gold translucentreflection film even with an optical disk using a chemically unstablematerial comparing to gold, such as silver or the like, as a translucentreflection film.

DISCLOSURE OF THE INVENTION

The inventors have made earnest investigations to solve the problembased on an estimation that deterioration, such as corrosion or thelike, of a translucent reflection film formed with an unstable materialcomparing to gold, such as silver, a silver alloy or the like, is causedby a slight electric current or the like due to humidity in an adhesivelayer, and as a result, it has been found that in the case where such anadhesive is used that contains a monomer having a solution resistancelarger than a certain value as a major component, the humidityresistance is improved, and excellent durability (reliability) can beobtained when a translucent reflection film formed with silver or asilver alloy is bonded, whereby the invention has been completed.Therefore, the invention relates to:

(1) an adhesive for an optical disk containing (A) a (meth)acrylatemonomer having a solution resistance of 1.0×10¹⁰ Ω·cm or more,

(2) the adhesive for an optical disk according to the above (1), whichcontains (B-1) a urethane (meth)acrylate and/or (B-2) a bisphenol typeepoxy (meth)acrylate, and (C) a photopolymerization initiator,

(3) the adhesive for an optical disk according to the above (1) or (2),which contains (D) a (meth)acrylate phosphate compound,

(4) the adhesive for an optical disk characterized in that a content ofa urethane (meth)acrylate (B-1) is 10% by mass or more but less than 50%by mass, the (meth)acrylate monomer (A) having a solution resistance of1.0×10¹⁰ Ω·cm or more is 40% by mass or more but less than 90% by mass,and, as the balance, the photopolymerization initiator (C) and,depending on necessity, a (meth)acrylate phosphate compound (D) and abisphenol A type epoxy (meth)acrylate (B-2) are contained, and a contentof said (meth)acrylate monomer (A) is larger than a content of theurethane (meth)acrylate (B-1),

(5) the adhesive for an optical disk according to any one of the above(1) to (4), wherein, said (meth)acrylate monomer (A) is at least oneselected from a C8 to C18 alkyl monoacrylate, tricyclodecanedimethyloldi(meth)acrylate and nonylphenoxyethyl acrylate,

(6) a bonded optical disk having a translucent reflection filmcomprising silicone, silver or a silver alloy, characterized by bondinga substrate having said translucent reflection film and an anothersubstrate with an adhesive of any one of the above (1) to (5), and

(7) the optical disk according to the above (6), wherein the bondedoptical disk is a DVD.

BEST MODE FOR CARRYING OUT THE INVENTION

The invention will be described in detail below. In the following, “%”and “part” are “part by mass” and “% by mass”, respectively, unlessotherwise indicated.

In the invention, the (meth)acrylate monomer (A) having a solutionresistance of 1.0×10¹⁰ Ω·cm or more is used, whereby damages of thetranslucent reflection film due to moisture can be reduced and thehumidity resistance of a cured product cured by ultraviolet irradiationis improved, and as a result, excellent durability (reliability) of theoptical disk can be thus obtained.

The adhesive of the invention is characterized by containing the(meth)acrylate monomer (A) having a solution resistance of 1.0×10¹⁰ Ω·cmor more, and the content thereof is generally 30% or more, preferably40% or more but less than 99%, and more preferably in a range of lessthan 90%. Examples of the (meth)acrylate monomer (A) include a C₈ to C₁₈alkyl monoacrylate, such as isooctyl acrylate, isodecyl acrylate, laurylacrylate, stearyl acrylate, tridecyl acrylate and the like, amonofunctional (meth)acrylate, such as isobornyl (meth)acrylate,nonylphenoxyethyl (meth)acrylate and the like, and a polyfunctional(meth)acrylate, such as tricyclodecanedimethylol di(meth)acrylate,5-ethyl-2-(2-hydroxy-1,1-dimethylethyl)-5-(hydroxymethyl)-1,3-dioxanedi(meth)acrylate and ditrimethylolpropane tetra(meth)acrylate and thelike. Particularly preferred examples of the (meth)acrylate monomer (A)include said C₈ to C₁₈ alkyl monoacrylate, tricyclodecanedimethyloldi(meth)acrylate, nonylphenoxyethyl acrylate and the like.

The (meth)acrylate monomer (A) may be used singly or in combination oftwo or more kinds thereof in arbitrary proportions. The content of thecomponent (A) in the adhesive may be in the aforementioned range, and itis preferably in a range of from 30 to 85%, more preferably from 35 to75%, further preferably from 40 to 75%, and most preferably from 50 to75%. However, in the case where the adhesive of the invention contains aphotopolymerization initiator and components to be added depending onnecessity, the upper limit is such a value that is obtained bysubtracting the contents thereof.

The adhesive of the invention contains the urethane (meth)acrylate (B-1)and/or the bisphenol type epoxy (meth)acrylate (B-2).

Specific examples of the urethane (meth)acrylate (B-1) include thefollowing polyols and a polyether series urethane acrylate obtained byreaction of an organic polyisocyanate and a hydroxyl group-containing(meth)acrylate compound. These are available as commercial products.Furthermore, it can be also obtained by reacting, in terms of molarratio of the foregoing raw materials, from 1 to 2 mole of the organicpolyisocyanate with from 0.1 to 2 mole of the hydroxyl group-containing(meth)acrylate compound, per 1 mole of the polyols, in the presence orabsence of an organic solvent. Among these, a polyether series urethaneacrylate having an aromatic ring (hereinafter, simply referred to as anaromatic urethane acrylate) obtained by using any one or both of anorganic polyisocyanate having an aromatic ring and a polyol having anaromatic ring is a preferred one.

Examples of the polyol used as a raw material of the (B-1) componentinclude a polyol having a carbon number of from 2 to 8 preferably havingfrom 2 to 4 hydroxyl groups, such as ethylene glycol, propylene glycol,diethylene glycol, 1,4-butanediol, neopentyl glycol, 1,6-hexanediol,3-methyl-1,5-pentanediol, cyclohexane-1,4-dimethylol and the like, abisphenol A poly(C2 to C4 alkoxy)diol, such as bisphenol Apolyethoxydiol, bisphenol A polypropoxydiol and the like, a polyol ofpoly(C1 to C5 alkylene)glycol, such as polyethylene glycol,polypropylene glycol, polybutylene glycol, polytetramethylene glycol,trimethylolpropane and the like, a polyester polyol obtained by reactingone kind or two or more kinds of these polyols with a polybasic acid,such as an aliphatic dicarboxylic acid having a carbon number of from 4to 8 and an aromatic polycarboxylic acid having an aromatic ring havinga carbon number of from 6 to 10 and having from 2 to 4 carboxyl groupson the aromatic ring, specifically, succinic acid, phthalic acid,isophthalic acid, terephthalic acid, adipic acid, azelaic acid and thelike, a polycaprolactone polyol obtained by reacting the polyol or thepolyester polyol with ε-caprolactone and a polycarbonate polyol.

Examples of the organic polyisocyanate used as a raw material of the(B-1) component include an alicyclic diisocyanate, preferably a C5 toC12 alicyclic diisocyanate having from 1 to 2 cyclic rings having acarbon number of from 5 to 6, such as isophorone diisocyanate,dicyclohexylmethane-4,4′-diisocyanate, dicyclopentanyl diisocyanate andthe like, an aromatic diisocyanate, preferably a C6 to C12 aromaticdiisocyanate having from 1 to 2 aromatic rings having a carbon number offrom 6 to 10, such as tolylene diisocyanate, xylylene diisocyanate andthe like, and an aliphatic diisocyanate, preferably an aliphaticdiisocyanate having a carbon number of from 5 to 10, such ashexamethylene diisocyanate, trimethylhexamethylene diisocyanate and thelike.

Examples of the hydroxyl group-containing (meth)acrylate used as a rawmaterial of the (B-1) component include a hydroxy(C1 to C8)aliphatic(meth)acrylate, such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl(meth)acrylate, 4-hydroxybutyl (meth)acrylate,cyclohexane-1,4-dimethylol mono(meth)acrylate and the like, and anε-caprolactone modified product thereof, such as ε-caprolactone-modified2-hydroxyethyl (meth)acrylate and the like.

The urethane (meth)acrylate (B-1) may be used singly or in combinationof two or more kinds thereof in the adhesive of the invention. Themolecular weight of the urethane (meth)acrylate (B-1) is preferably from400 to 10,000. Examples of the urethane (meth)acrylate (B-1) include anaromatic urethane acrylate UA-937 (a trade name) produced by NipponKayaku Co., Ltd.

The content of the (B-1) component in the adhesive is not particularlylimited as far as the effect of the invention is not impaired. It isgenerally in a range of from 1 to 70%, and preferably in a range of from5 to 50%. However, in the case where the adhesive of the inventioncontains a photopolymerization initiator and components to be addeddepending on necessity, the upper limit is such a value that is obtainedby subtracting the contents thereof. In some cases, the content of the(B-1) component is preferably 10% or more, and more preferably 25% ormore, and the upper limit may be in a range of less than 65%, and morepreferably less than 50%, depending on the kind and the content of the(A) component and the kind of the (B-1) component to be used incombination, and the like.

In the adhesive of the invention, the balance other than the(meth)acrylate monomer (A) component and the urethane (meth)acrylate(B-1) component contains at least the photopolymerization initiator (C),and may further contain, depending on necessity, any one or both of thebisphenol type epoxy (meth)acrylate (B-2) and the (meth)acrylatephosphate compound (D), and (E) a (meth)acryloyl group-containingcompound other than those in the foregoing and other additives.Therefore, in the case where there is no space for containing componentsother than the (meth)acrylate monomer (A) component and the urethane(meth)acrylate (B-1) component due to the lower limits and the upperlimits of these components, the upper limits are decreased such amountsthat correspond to the contents of the photopolymerization initiator (C)and the components added depending on necessity.

Specific examples of the bisphenol type epoxy (meth)acrylate (B-2)include an epoxy (meth)acrylate obtained by reacting a bisphenol A typeepoxy resin, such as Epikote 828, 1001 and 1004 and the like, and abisphenol F type epoxy resin, such as Epikote 4001, 4002 and 4003,produced by Yuka Shell Epoxy Co., Ltd., and the like, with (meth)acrylicacid, and those using a bisphenol A type epoxy resin are preferred. Thebisphenol type epoxy (meth)acrylate (B-2) may be used singly or incombination of two or more kinds thereof in the adhesive of theinvention. The using amount in the adhesive is preferably from 0 to 50%,and particularly preferably from 0 to 40%.

Specific examples of the photopolymerization initiator (C) used in theinvention include 2,2-dimethoxy-1,2-diphenylethan-1-one,2-methyl-1-(4-(methylthio)-phenyl)-2-morpholinopropan-1-one,2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butanone-1,bis(2,6-dimethoxybenzoyl)-2,4,4-trimethyl-pentylphosphine oxide,bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide,2,4,6-trimethylbenzoyldiphenylphosphine oxide,2-hydroxy-2-methyl-1-phenylpropan-1-one, 1-hydroxycyclohexyl phenylketone and the like. The component (C) may be used singly or incombination of two or more kinds thereof in the adhesive of theinvention. The content in the adhesive is generally from 0.2 to 20%,preferably from 0.5 to 20%, and more preferably from 0.5 to 10%, and maybe from 1 to 10% in some cases.

In the adhesive of the invention, the (meth)acrylate phosphate compound(D) is preferably used for improving the adhesiveness. Specific examplesof the (meth)acrylate phosphate compound (D) may be a monoester, adiester and a triester, as far as they are (meth)acrylates having aphosphoric ester structure, and include mono(oxyethyl methacrylate)phosphate, tri(oxyethyl methacrylate) phosphate and the like. The(meth)acrylate phosphate compound (D) may be used singly or incombination of two or more kinds thereof in the adhesive of theinvention. The content in the adhesive is about from 0 to 5%, preferablyfrom 0.0001 to 3%, and more preferably from 0.001 to 3%, and is alsopreferably from 0.05 to 3%.

In the adhesive of the invention, the (meth)acryloyl group-containingcompound (E) other than those in the foregoing may be used incombination for improving the performance. Examples thereof includephenyloxyethyl (meth)acrylate, phenyloxyethyloxyethyl (meth)acrylate,tetrahydrofurfuryl (meth)acrylate, dicyclopentenyloxyethyl acrylate,ethylcarbitol (meth)acrylate, hydroxypivalic acid neopentyl glycoldi(meth)acrylate, triethylene glycol monomethyl ether (meth)acrylate,4-hydroxybutyl (meth)acrylate, tricyclo(5,2,1,0,2,6)decanyl(meth)acrylate, 1,6-hexanediol di(meth)acrylate, bisphenol Apoly(n=about 4)ethoxylate di(meth)acrylate, tripropylene glycoldi(meth)acrylate, polyethylene glycol di(meth)acrylate,trimethylolpropane tri(meth)acrylate and the like. These (meth)acryloylgroup-containing compound may be used singly or in combination of two ormore kinds thereof in the adhesive of the invention. The content in theadhesive is about from 0 to 30%.

In the adhesive of the invention, other additives may be used incombination, such as a silane coupling agent, leveling agent, apolymerization inhibitor, a light stabilizer, an antioxidant, anantistatic agent, a surface lubricant, a filler and the like.

The adhesive of the invention can be obtained by mixing and dissolvingthe components at from ordinary temperature to 80° C. The cured productof the invention can be obtained by applying the adhesive of theinvention to a substrate to be adhered, and then irradiating with light,such as an ultraviolet ray, a visible ray and the like, according to anordinary procedure.

Preferred solution properties of the adhesive of the invention are aviscosity measured with a B type viscometer at 25° C. of from 100 to 700mpa·S and an acid value of from 0.01 to 5 (mgKOH/g), and the curedproduct preferably has a refractive index (25° C.) of from 1.48 to 1.58and a gel fraction of from 70 to 100%.

Upon curing the adhesive of the invention, the adhesive may beirradiated by light of from ultraviolet to near ultraviolet after beingapplied to a substrate. A light source is not limited as far as thelight can be irradiated. Examples thereof include a low pressure, highpressure or very high pressure mercury lamp, a metal halide lamp, a(pulse) xenon lamp, an electrodeless lamp and the like.

The bonded optical disk according to the invention can be obtained inthe following manner.

For example, the adhesive of the invention is applied on an optical disksubstrate, to which aluminum or the like has been sputtered, atranslucent reflection film, such as silver and the like, is placed onthe sputtered optical disk substrate with no air being introduced intothe adhesive, followed by being bonded by such a method as spin coatingor the like, and then light of from ultraviolet to near ultraviolet isirradiated from one side or both sides to cure and adhere, so as toobtain the bonded optical disk according to the invention.

In the case where an optical disk substrate having a translucentreflection film formed with a material, such as silicone, silver, asilver alloy and the like, that is unstable comparing to gold is bondedto another optical disk substrate with the adhesive of the invention, itis preferred that the film thickness of the adhesive layer is from 1 to100 μm, and preferably from 40 to 70 μm. The application method is notlimited as far as such conditions are satisfied. Examples thereofinclude a spin coating method, a 2P method, a roll coating method, ascreen printing method and the like.

EXAMPLE

The invention will be described in more detail with reference toexamples below.

Ultraviolet ray-curable adhesives having compositions shown in Table 1were mixed and dissolved to prepare adhesives according to theinvention.

The codes for the compositions shown in the table are as follows.

UA-937: aromatic urethane acrylate, produced by Nippon Kayaku Co., Ltd.BP-4EA: bisphenol A poly(n = about 4)ethoxylate diacrylate, produced byKyoeisha Chemical Co., Ltd. 4EG-A: tetraethylene glycol diacrylate,produced by Kyoeisha Chemical Co., Ltd. R-684: tricyclodecanedimethyloldiacrylate, produced by Nippon Kayaku Co., Ltd. SR-489: tridecylacrylate, produced by Sartomer Company Viscoat #150: tetrahydrofurfurylacrylate, produced by Osaka Organic Chemical Industry, Ltd. 4HBA:2-hydroxybutyl acrylate, produced by Nippon Kasei Chemical Co., Ltd.PM-2: bis (oxyethyl methacrylate) phosphate, produced by Nippon KayakuCo., Ltd. Irgacure 184: 1-hydroxycyclohexyl phenyl ketone, produced byCiba Specialty Chemicals, Inc., photopolymerization initiator

The monomer components have the following solution resistance values(Ω·cm).

Universal Electrometer (produced by Kawaguchi Electric Works Co., Ltd.)was used for the measurement of the solution resistance.

Name of Component Solution resistance value (Ω · cm) BP-4EA 8 × 10⁹ Ω ·cm 4EG-A 7.9 × 10⁸ Ω · cm R-684 1.2 × 10¹¹ Ω · cm SR-489 5.0 × 10¹¹ Ω ·cm Viscoat #150 5.8 × 10⁸ Ω · cm 4HBA 7.5 × 10⁸ Ω · cm

TABLE 1 Comparative Example Example 1 2 3 1 2 UA-937 30 35 30 36 40BP-4EA — — — 7 5 4EG-A — — — 8 28 R-684 57 42 57 20 — SR-489 15 20 — — —Viscoat #150 — — 15 — — 4HBA — — — 24 19 PM-2 (×10⁻³) 3 3 3 3 3 Irgacure184 0.7 0.7 0.7 0.7 0.7 Viscosity 443 479 350 547 653 (mpa · S/25° C.)Acid value (mgKOH/g) 0.3 0.3 0.3 0.3 0.3 Reflection film Au Ag Au Ag AuAg Au Ag Au Ag After 500 hours ∘ ∘ ∘ ∘ ∘ ∘ ∘ x ∘ x (visual observation)Note: All the compositional proportions in the table are in terms ofpart by mass.

The compositions shown in Table 1 thus obtained were subjected toadhesion in the following steps 1 to 3.

1. 25 g of the adhesive was fed to a circular form on an insideperiphery of an aluminum-sputtered DVD substrate.

2. A silver translucent reflection film (Ag) was placed on the sputteredDVD substrate with no air being introduced in to the adhesive, followedby being bonded by spin coating at 2,000 rpm for 4 seconds. A goldtranslucent reflection film (Au) was also bonded to the sputtered DVDsubstrate under the same conditions.

3. Irradiation was made from the side of the silver or gold translucentreflection film by a high pressure mercury lamp (80 W/cm) at 300 mJ/cm²to cure and adhere.

After the adhesion, the DVD substrates to which the silver or goldtranslucent reflection film had been adhered were allowed to stand undercircumstances of 80° C. and 85% RH for 500 hours. The state of thereflection film was visually observed. The results of the observationwere indicated in terms of the following in Table 1.

O: No change was observed in the reflection film.

Δ: Slight discoloration or pin holes were observed in the reflectionfilm.

x: Significant discoloration or pin holes were observed in thereflection film.

It is apparent from the evaluation results in Table 1 that the adhesiveof the invention exerts less influence particularly on a silvertranslucent reflection film, and is excellent in durability.

INDUSRIAL APPLICABILITY

The adhesive of the invention provides such high durability(reliability) in a bonded optical disk using a silver or silver alloytranslucent reflection film that is equivalent to the conventionalbonded optical disk using gold as a translucent reflection film.Therefore, the adhesive of the invention is significantly useful as anadhesive for an optical disk applicable to increased recordingcapacities.

1. An adhesive for an optical disk containing (A) a (meth)acrylatemonomer having a solution resistance of 1.0×10¹⁰ Ω·cm or more.
 2. Theadhesive for an optical disk according to claim 1, which contains (B-1)a urethane (meth)acrulate and/or (B-2) a bisphenol type epoxy(meth)acrylate, and (C) a photopolymerization initiator.
 3. The adhesivefor an optical disk according to claim 1 or 2, which contains (D) a(meth)acrylate phosphate compound.
 4. The adhesive for an optical diskcharacterized in that a content of a urethane (meth)acrylate (B-1) is10% by mass or more but less than 50% by mass, the (meth)acrylatemonomer (A) having a solution resistance of 1.0×10¹⁰ Ω·cm or more is 40%by mass or more but less than 90% by mass, and, as the balance, thephotopolymerization initiator (C) and, depending on necessity, the(meth)acrylate phosphate compound (D) and the bisphenol type epoxy(meth)acrylate (B-2) are contained, and a content of the (meth)acrylatemonomer (A) is larger than a content of the urethane (meth)acrylate(B-1).
 5. The adhesive for an optical disk according to any one of claim1 to 4, wherein the (meth)acrylate monomer (A) is at least one selectedfrom a C8 to C18 alkyl monoacrylate, tricyclodecanedimethyloldi(meth)acrylate and nonylphenoxyethyl acrylate.
 6. An optical diskhaving a translucent reflection film comprising silicone, silver or asilver alloy, characterized in that the optical disk is a bonded opticaldisk comprising a substrate having the translucent reflection filmbonded to another substrate with an adhesive of any one of claim 1 to 5.7. The optical disk according to claim 6, wherein the bonded opticaldisk is a DVD.